1. Field of the Invention
The present invention relates to a photoelectric conversion device wherein a photoelectric conversion layer composed of a crystalline silicon film is formed on a substrate, and a process for producing the same.
2. Related Art
A photoelectric conversion device can be produced using any one of various semiconductor materials and organic compound materials. However, the photoelectric conversion device is industrially produced using silicon mainly. The photoelectric conversion device using silicon can be classified into a bulk type photoelectric conversion device using a wafer made of monocrystal silicon or polycrystal silicon and a thin film type photoelectric conversion device wherein a silicon film is formed on a substrate. For the bulk type photoelectric conversion device, a semiconductor substrate (such as a silicon wafer) is necessary in the same way as for a LSI (large-scale integrated circuit). The production amount thereof is limited by the supply capacity of the semiconductor substrate. On the other hand, it is considered that potential production capacity of the thin film type photoelectric conversion device is higher because of the use of a semiconductor film on a given substrate.
At present, a photoelectric conversion device using amorphous silicon is made practicable. However, this photoelectric conversion device has lower conversion efficiency than the photoelectric conversion device using monocrystal silicon or polycrystal silicon. Furthermore, this photoelectric conversion device has problems such as deterioration by light. Thus, the use of this photoelectric conversion device is limited to products having a small power consumption, such as a pocket calculator. For sunshine power generation, photoelectric conversion devices using a silicon film obtained by crystallizing an amorphous silicon film (the obtained silicon film being referred to as a crystalline silicon film hereinafter) have been actively developed.
The method of forming the crystalline silicon film is classified into melting recrystallization and solid phase growth methods. In both the methods, amorphous silicon is formed on a substrate, and this silicon is recrystallized to form a crystalline silicon film. In either case, the substrate is required to endure the crystallization temperature of silicon. Thus, the material which can be used for the substrate is limited. Particularly in the melting recrystallization method, the material for the substrate is limited to a material enduring the melting point of silicon, that is, 1412° C.
The solid phase method is a method of forming an amorphous silicon film on a substrate and then subjecting the film to heat treatment to crystalline the film. Usually, the amorphous silicon film is hardly crystallized at a temperature of 500° C. or lower. Practically, it is necessary to heat the amorphous silicon film at 600° C. or higher. For example, in the case that an amorphous silicon film formed by a vapor growth method is heated to be crystallized, a heating time of 10 hours is necessary when heating temperature is 600° C. When the heating temperature is 550° C., a heating time of 100 hours or more is necessary.
For the reasons as described above, the substrate for forming a crystalline silicon film is required to have high heat-resistance. It is therefore preferred to use quartz, carbon, a ceramic material or the like as the material for the substrate. However, such a substrate is not necessarily suitable for a reduction in production costs. It would be primarily desired that an inexpensive material circulated in a great amount in the market is used as the material for the substrate. However, for example, a #7059 glass substrate made by Corning Incorporated, which is in general frequently used, has a strain point of 593° C. Thus, if a conventional crystallizing technique is used, this substrate is distorted to generate large deformation. Therefore, the substrate is not used. Since the substrate is made of a material essentially different from silicon, a monocrystal film cannot be obtained even if heat treatment for crystallization is merely performed. As a result, only a polycrystal film can be obtained. The grain size of the polycrystal film is not easily made large. This fact results in the suppression of an improvement in the efficiency of photoelectric conversion device.
As a method for solving the above-mentioned problems, JP-A-7-58338 discloses a technique wherein a very small amount of a catalyst element is added as a catalyst material for the promotion of crystallization at low temperature, thereby attaining the crystallization. According to this official gazette open to the public, it becomes possible to make heat treatment temperature low and make treatment time short. For example, in the case that the heating temperature is set to 550° C., it is verified that silicon is crystallized by heat treatment for 4 hours. The official gazette states that a single element of nickel (Ni), iron (Fe), cobalt (Co) or platinum (Pt), a compound of any one of them and silicon, or the like is suitable for the catalyst element.
Originally, however, all of the catalyst materials used to promote the crystallization are materials unpreferable for crystalline silicon. It is therefore desired that the concentration of the catalyst material is as low as possible after the crystallization. The concentration of the catalyst material necessary for promoting the crystallization is a range from 1×1017 to 1×1020/cm3. However, even if the concentration is relatively low, the element suitable for the catalyst material, when taken in silicon, generates a defect level because the element is a metal. Thus, it is evident that this defect level causes the deterioration of important characteristics for a photoelectric conversion device, such as the lifetime of carriers.
Incidentally, it can be considered that the outline of the action principle of a photoelectric conversion device produced by forming a PN junction is as follows. The photoelectric conversion device absorbs light, and generates carriers (i.e., electrons and holes) by the energy of the absorbed light. About the generated carriers, the electrons move toward its n layer and the holes move toward its p layer by drift and diffusion based on an electric field. In the case that silicon has many defect levels, the carriers are trapped into the defect levels on their way to become extinct. That is, the photoelectric conversion characteristic of the photoelectric conversion device deteriorates. The time from the generation of the electrons and holes to the extinction thereof is called a lifetime. It is desired that this value is larger for the photoelectric conversion device. Therefore, it is necessary that the amount of impurity elements, which generate the defect level in silicon, are originally as small as possible.